An episode selection procedure was developed and applied to select sets of days representing characteristic meteorol. conditions leading to high ozone episodes over the Swiss Plateau. The selection procedure was applied to data extending from Jan. 1991 through Dec. 1998, and is comprised of two steps: First, days were classified according to obsd. air quality and meteorol. characteristics using classification and regression trees anal. (CART). Second, the CART results were used in conjunction with obsd. air quality data to identify sets of days characteristic of those leading to elevated ozone. These sets of days were selected to optimize how well a limited no. of days represented seasonal air quality, and that formed longer episodes for use in the air quality modeling. CART anal. was performed for three zones of the Swiss Plateau that have different air quality and meteorol. characteristics. The results for two zones were used together in the episode selection procedure in order to identify days representative for the whole Plateau. Meteorol. anal. for a third zone suggested that it would be strongly impacted by pollutants transported in from outside the country. One thousand and eight hundred optimization runs were performed to minimize the likelihood that the set of days was a local optimum, increasing the robustness for use in air quality modeling anal. Fifteen days, grouped in four episodes ranging from 3 to 5 days were selected along with their calcd. representativeness (or wt.) to recreate a seasonal metric. The variety of local as well as regional meteorol. characteristics showed that the episode selection procedure chose days representing a diverse set of meteorol. situations which are assocd. with elevated ozone. This set of episodes can now be used to test air quality strategies.